Adhesive for difficult to bond surfaces

ABSTRACT

Improved aqueous emulsion vinyl acetate-ethylene (VAE) polymer adhesives for use in packaging applications and on difficult to bond surfaces such as polyethylene, poly(ethylene terephthalate), and oriented polypropylene. The VAE polymers contain about 55-80 wt % vinyl acetate, 15-45 wt % ethylene, and 0-30 wt % of one or more additional ethylenically unsaturated copolymerizable monomer, based on the total weight of monomers. Cast films of the VAE emulsion polymers of this invention should have a tensile storage modulus (test frequency of 6.28 rad/sec) within the area encompassed by the following data points: 1×10 5  and 2×10 7  dynes/cm 2  at 23° C., and 1×10 3  and 2×10 6  dynes/cm 2  at 70° C.; preferably within the area encompassed by the following data points: 1×10 6  and 1.5×10 7  dynes/cm 2  at 23° C., and 3×10 4  and 9×10 5  dynes/cm 2  at 70° C.

BACKGROUND OF THE INVENTION

There are many known aqueous emulsion polymers that are used in avariety of applications, including adhesive applications. Vinylacetate-ethylene (VAE) emulsion polymers have been preferred inwater-based packaging adhesive applications primarily because of the lowcost of production, availability of reactants, the adhesion, settingspeed, and wet tack properties they offer on glass, metal, corrugatedand other paperboard materials, and the ease at which they can beapplied and maintained on adhesive application equipment.

Over the past 8-10 years there has been a continuous shift in the typeof packaging materials utilized in the industry. Today the segment whichis based on plastic or polymeric materials is growing and rapidlyreplacing the traditional materials mentioned above. This new class ofplastic packaging is far more difficult to adhere to than glass, metaland paper-based packaging materials which have been in use.

Various methods have been used to improve the bonding of VAE adhesivesto difficult to bond substrates, such as polyethylene, polypropylene,poly(ethylene terephthalate), and surfaces having low surface energy,low polarity, or little or no porosity. Surface treatments, such asplasma treatment, corona discharge, or chemical oxidation, have beenused to alter the surface so that they can more easily be bonded withVAE adhesives; however these treatments are expensive and timeconsuming, and are not always possible.

In order to achieve adhesion on difficult to bond surfaces, traditionalVAE adhesive formulations have been highly plasticized or formulated.However, introduction of additional compounding aids adds to thecomplexity and cost of the formulations and, frequently leads toproblems during the application and/or machining of the resultingadhesives. In addition, the list of additives available to the adhesiveformulators is decreasing because many of the solvents and plasticizersused in the past are no longer environmentally acceptable.

Another approach to improve the bonding of VAE adhesives to low polaritysurfaces has been to add other monomers to the polymer. For example,U.S. Pat. No. 5,371,137 (Blincow et al. 1994) discloses VAE copolymeremulsions to which about 5% to about 85% of vinyl esters of C₄ to C₁₈primary or secondary carboxylic acids have been added as a monomer. U.S.Pat. No. 5,500,251 (Burgoyne et al., 1996) disclose the incorporation ofthe following compounds into VAE systems to promote adhesion to lowenergy polyolefin surfaces: N-(4-alkylphenyl)acrylamides,N-(4-alkylphenyl)methacrylamides and N-(4-alkylphenyl)maleimides.

VAE pressure sensitive adhesives will adhere to most surfaces but lackof both cohesive strength and wet tack limits the use of these adhesiveson difficult to bond surfaces, particularly when the surfaces areexposed to high temperatures. U.S. Pat. No. 4,128,518 (Oyamada et al.)discloses a VAE pressure sensitive adhesive containing a base materialof an aqueous emulsion of vinyl acetate-ethylene copolymer having anethylene content of 15 to 40% by weight, benzene-insoluble part of lessthan 30% by weight, and an intrinsic viscosity of the benzene-solublepart of 0.4 to 1.4 dl/g. It is prepared by the emulsion copolymerizationof vinyl acetate and ethylene and optionally at least one unsaturatedmonovinyl monomer in the presence of a protective colloid and apolyoxyethylenic nonionic surfactant.

With increased use of difficult to bond or low polar surfaces, such aspolyethylene and oriented polypropylene, in the packaging industry,there has been continuing interest in producing an adhesive thatprovides good adhesion without the need to pretreat the surfaces and/oradd plasticizers or other modifiers to VAE adhesives.

BRIEF SUMMARY OF THE INVENTION

This invention is directed to improved vinyl acetate-ethylene (VAE)latex polymers, that are suitable for use as adhesives for packagingapplications and on difficult to bond surfaces such as polyethylene(PE), poly(ethylene terephthalate) (PET), metallized poly(ethyleneterephthalate) (MPET), and oriented polypropylene (OPP). They havespecific viscoelastic properties as indicated by tensile storage modulusof the cast film. The tensile storage modulus, at a test frequency of6.28 rad/sec, is defined by the area encompassed by the following datapoints: 1×10⁵ and 2×10⁷ dynes/cm² at 23° C., and 1×10³ and 2×10⁶dynes/cm² at 70° C. The VAE latex polymer contains 55 to 80 wt % vinylacetate, 15 to 45 wt % ethylene, and 0 to 30 wt % of one or moreadditional ethylenically unsaturated copolymerizable monomer.

The advantages of the latexes of this invention are:

they can be applied directly to difficult to bond surfaces withoutpretreating the surfaces or adding modifiers to the latex;

they have excellent adhesive properties on difficult to bond surfaces;and

they are useful for packaging applications.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a graph of tensile storage modulus vs temperature for severalVAE aqueous emulsion polymer adhesives.

FIG. 2 is a magnified section of the graph presented in FIG. 1, showingthe desired (solid line) and preferred (broken line) region of tensilestorage modulus of the polymers of this invention.

DETAILED DESCRIPTION OF THE INVENTION

The aqueous emulsion polymers according to this invention are useful asadhesives on difficult to bond surfaces and in packaging applications.The polymers comprise vinyl acetate, ethylene, and, optionally, one ormore additional ethylenically unsaturated copolymerizable monomer.

Specifically, the emulsion polymers comprise 55-80 wt % vinyl acetate,15-45 wt % ethylene, and 0-30 wt % of one or more additionalethylenically unsaturated copolymerizable monomer; preferably thecopolymers contain 60 to 75 wt % vinyl acetate, 20 to 40 wt % ethylene,and 1-5 wt % of one or more additional ethylenically unsaturatedcopolymerizable monomer, based on total weight of monomers.

The additional ethylenically unsaturated copolymerizable monomer can beC₃ -C₁₀ alkenoic acids, such as acrylic acid, methacrylic acid, crotonicacid and isocrotonic acid and their esters with C₁ -C₁₈ alkanols, suchas methanol, ethanol, propanol, butanol, and 2-ethylhexanol; vinylhalides, such as vinyl chloride; alpha, beta-unsaturated C₄ -C₁₀alkenedioic acids such as maleic acid, fumaric acid, and itaconic acidand their monoesters and diesters with the same C₁ -C₁₈ alkanols; andnitrogen containing monoolefinically unsaturated monomers, particularlynitriles, amides, N-methylol amides, lower alkanoic acid ethers ofN-methylol amides and allylcarbamates, such as acrylonitrile,acrylamide, methacrylamide, N-methylol acrylamide, N-methylolmethacrylamide, N-methylol allylcarbamate, and lower alkyl ethers orlower alkanoic acid esters of N-methylol acrylamide, N-methylolmethacrylamide and N-methylol allylcarbamate. A carboxyl-containingmonomer, such as acrylic acid, is preferred.

Cast films of the VAE emulsion polymers of this invention should have atensile storage modulus (test frequency of 6.28 rad/sec) within the areaencompassed by the following data points: 1×10⁵ and 2×10⁷ dynes/cm² at23° C., and 1×10³ and 2×10⁶ dynes/cm² at 70° C.; preferably within thearea encompassed by the following data points: 1×10⁶ and 1.5×10⁷dynes/cm² at 23° C., and 3×10⁴ and 9×10⁵ dynes/cm² at 70° C. It has beenfound in this invention that tensile mechanical properties, specificallytensile storage modulus, provide an accurate measure of viscoelasticproperties and is an important indicator in evaluating adhesiveproperties.

Without intending to be bound by theory, it is believed that VAEemulsion polymers having a tensile storage modulus below the areaencompassed by 1×10⁵ and 2×10⁷ dynes/cm² at 23° C., and 1×10³ and 2×10⁶dynes/cm² at 70° C., will be too soft for use as an adhesive fordifficult to bond surfaces, and VAE emulsion polymers having a tensilemodulus above the area will not have sufficient peel strength fordifficult to bond surfaces.

The method of producing the latex polymers of this invention is believedto be an important factor in producing an adhesive with tensilemechanical properties that provide excellent peel strength whileretaining sufficient creep resistance to make it useful for applicationto difficult to bond surfaces.

Below are described process conditions which have been found to beeffective in producing an aqueous emulsion polymer having the requiredtensile storage modulus. Process conditions that are considered to beparticularly important are: controlling the amount of vinyl acetate inthe reaction medium at the beginning and during the polymerizationprocess, adding initiator at the high end of the amounts typically usedin emulsion polymerization reactions, and use of a chain transfer agent.It is believed that these conditions serve to control the polymermolecular weight and ultimately the viscoelastic properties as shown bytensile dynamic mechanical data.

The reaction vessel is charged initially with less than 15%, preferablyless than 10%, vinyl acetate, and the remaining vinyl acetate is delayfed (i.e. added during polymerization) so that the unreacted vinylacetate concentration is maintained below about 5%, preferably below3.5%, based on total monomers, during the polymerization process. Vinylacetate monomer is added at a rate that limits the polymerization timeto no longer than about 10 hours, preferably less than 6 hours. Shortreaction times, i.e., less than 6 hours, are preferred in order tomaximize throughput during production and to improve productperformance.

The quantity of ethylene entering into the copolymer is influenced byunreacted vinyl acetate, pressure, agitation and viscosity of thepolymerization medium. Thus, to increase the ethylene content of thecopolymer, high pressures, greater agitation and a low viscosity can beemployed. Ethylene pressure ranges from about 500 psig to 1400 psig (34to 95 atm), preferably about 1000 psig (68 atm).

Polymerization can be initiated by thermal initiators or by a redoxsystem. A thermal initiator is typically used at temperatures at orabove about 70° C. and redox systems are preferred at temperatures belowabout 70° C. The amount of thermal initiator used in the process is 0.1to 3 wt %, preferably more than about 0.5 wt %, based on total monomers.Thermal initiators are well known in the emulsion polymer art andinclude, for example, ammonium persulfate, sodium persulfate, and thelike. The amount of oxidizing and reducing agent in the redox system isabout 0.1 to 3 wt %. Any suitable redox system known in the art can beused; for example, the reducing agent can be a bisulfite, a sulfoxylate,ascorbic acid, erythorbic acid, and the like. The oxidizing agent caninclude hydrogen peroxide, organic peroxide such as t-butyl peroxide,persulfates, and the like.

Chain transfer agents, well known in the aqueous emulsion polymerizationart are typically used but are not required. Examples include dodecylmercaptan, mercaptocarboxylic acids, and esters of mercaptocarboxylicacid. The chain transfer agent is added at levels of about 0.02 to 2 wt%, preferably 0.1 to 1 wt %, based on the weight of monomers.

Effective emulsion polymerization reaction temperatures range from about50° and 100° C.; preferably, 75° to 90° C.

In addition to the above reaction conditions and components, the polymerlatex may be stabilized with conventional emulsifiers and protectivecolloids; however, the use of polyvinyl alcohol is preferred because itimproves heat resistance, adhesion to metallized surfaces (i.e. MPET),speed of set, and wet tack. The stabilizing system typically consists of0.5-5 wt %, preferably 2-5 wt %, of polyvinyl alcohol and 1-4 wt %,preferably 1.5-3 wt % of a surfactant, based on vinyl acetate monomer.The polyvinyl alcohol that is used in the stabilizing system can be75-99+ mole % hydrolyzed, preferably 85-90, and especially 87-89 mole %hydrolyzed, and has a degree of polymerization ranging from 50 to 3000;preferably, 100 to 1500; and most preferably, 200 to 1000. The degree ofpolymerization of the polyvinyl alcohol affects the viscosity of theemulsion product; i.e., as degree of polymerization increases, viscosityof the emulsion product increases. In this emulsion polymer, a viscositybetween about 2,000 and 4,000 cps is preferred for ease of handling theemulsion product.

The stabilizer can also contain a surfactant at a level of about 1-4 wt%, preferably 1.5-3 wt %, based on vinyl acetate monomer. Thesurfactants contemplated for the invention include any of the known andconventional surfactants and emulsifying agents, principally thenonionic and anionic materials, heretofore employed in the emulsioncopolymerization of vinyl acetate and ethylene; polyalkoxylatedsurfactants being especially preferred. Among the nonionic surfactantsfound to provide good results are the Igepal surfactants supplied byRhone-Poulenc. The Igepal surfactants are members of a series ofalkylphenoxy-poly(ethyleneoxy)ethanols having alkyl groups containingfrom about 7-18 carbon atoms, and having from about 4 to 100 ethyleneoxyunits, such as the octylphenoxy poly(ethyleneoxy)ethanols, nonylphenoxypoly(ethyleneoxy)ethanols, and dodecylphenoxy poly(ethyleneoxy)ethanols.Examples of nonionic surfactants include polyoxyalkylene derivatives ofhexitol (including sorbitans, sorbides, manitans, and mannides)anhydride, partial long-chain fatty acid esters, such as polyoxyalkylenederivatives of sorbitan monolaurate, sorbitan monopalmitate, sorbitanmonostearate, sorbitan tristearate, sorbitan monooleate and sorbitantrioleate.

The glass transition temperature (Tg) of the aqueous emulsion polymersof this invention is typically 0° C. or lower; preferably -5° to -45°C., because they have both good flexibility and good adhesion.

Although not all inclusive, examples of difficult to bond surfaces arepolyethylene (PE), poly(ethylene terephthalate) (PET), metallizedpoly(ethylene terephthalate) (MPET), polypropylene, orientedpolypropylene (OPP), polyester, aluminum foil, and coated paperboard.Included among the difficult to bond surfaces are surfaces having asurface energy of less than about 40 dynes/cm².

The invention will be further clarified by a consideration of thefollowing examples, which are intended to be purely exemplary of theinvention.

Examples 1-7 illustrate the production of VAE polymer emulsions usingthe process conditions described above. Examples 1A and 1B were 30gallon scale-ups of Example 1.

EXAMPLE 1 Vinyl Acetate-Ethylene-Acrylic Acid Polymers

The polymer was prepared in a one-gallon reactor equipped with a jacketfor cooling, a mechanical turbine agitator, and metering pumps foraddition of the various feeds. Deionized water was used. The reactor wascharged with 1113 g of water, 57.0 g of Airvol® 203 poly (vinyl alcohol)(supplied by Air Products and Chemicals, Inc.), 40.7 g of Igepal CO-887(70% solution of a 30 mole nonylphenolethoxylate supplied byRhone-Poulenc) and 75.0 g vinyl acetate. After the initial charging, thereactor was purged with nitrogen followed by an ethylene purge, andheated under agitation to 85° C., then pressurized to 1000 psig (68 atm)with ethylene. Addition of an ammonium persulfate (40 g ammoniumpersulfate (APS), 17 g sodium bicarbonate and 347 g water) solution wasbegun at a rate of 2.5 g/min; after 8 minutes the feed rate was slowedto 2.0 g/min. Upon evidence of an exotherm (about 5 minutes afterbeginning the persulfate feed), addition of a second monomer solutionwas begun and added uniformly over a 3-hour period. The second monomersolution consisted of: 1225.2 g vinyl acetate, 57.0 g acrylic acid, and9.5 g n-dodecyl mercaptan, as chain transfer agent. During thepolymerization, ethylene was introduced to the reactor at a ratesufficient to maintain the pressure at 1000 psig. The persulfateaddition was discontinued 15 minutes after the monomer addition wascomplete. The contents were then held at 85° C. for an additional 45minutes and then cooled to 32° C. The contents were then transferred toa 3-gallon vessel where vacuum was used to remove any unreactedethylene. At this point, 2 g of Colloids™ 675 (a proprietary defoamercomposition supplied by Rhone-Poulenc) was added to reduce foaming,followed by 1 g of sodium erythorbate in 10 g of water, then 1 g oft-butyl hydroperoxide in 10 g of water, to reduce residual vinylacetate.

EXAMPLE 1A Scale-Up of Example 1

Example 1A was a scale-up of Example 1, from a one-gallon reactor to a30-gallon reactor. In this example, Airvol 203 was replaced withAirvol®205 poly (vinyl alcohol), supplied by Air Products and Chemicals,Inc. The following additional changes were made: pressure was 900 psi(61 atm); ammonium persulfate was 1.5 wt % of the total monomers.

EXAMPLE 1B Scale-Up of Example 1

Example 1B was a scale-up of Example 1 from a one-gallon reactor to a30-gallon reactor. In this example, Airvol 203 was replaced withAirvol205 and Airvol®523 (86.3 and 13.7%, respectively) poly (vinylalcohol), supplied by Air Products and Chemicals, Inc. The followingadditional changes were made: pressure was 900 psi (61 atm); thetemperature was 80° C.; initial vinyl acetate was increased from 5.5 to8.4% of the total monomer charge. The amount of ammonium persulfate was1.1 wt % of total monomers.

EXAMPLES 2-4

The procedure of Example 2 was the same as Example 1, except that 1/2the amount of chain transfer agent (3.8 g) was used. Examples 3 and 4were the same as Example 2 with the following changes in the amount ofreactants and the pressure:

Example 3: the pressure was 700 psi (48 atm) and 1525.1 g vinyl acetatewas used in the second monomer solution.

Example 4: the pressure was 550 psi (37 atm) and 1650.0 g vinyl acetatewas used in the second monomer solution.

EXAMPLE 5 No Carboxyl Monomer

The procedure of Example 2 was followed, with the following differences.The stabilizing system consisted of 1187 g water, 54.1 g Airvol 203, and38.7 g of Igepal. The delay feed of vinyl acetate was 1503.4 g.

EXAMPLES 6 and 7 No Chain Transfer Agent and Amount of Initiator wasReduced

The procedure of Example 2 was followed with the following exceptions:

Example 6: Initiator was reduced from 40 to 24.5 g of ammoniumpersulfate and from 17 to 13.6 g of sodium bicarbonate in 370 g ofwater, pressure was 1050 psi (71 atm), and no n-dodecyl mercaptan wasadded.

Example 7: Initiator was reduced to 12.6 g of ammonium persulfate and7.0 g sodium bicarbonate in 400 g of water, pressure was 1150 psi (78atm), and no n-dodecyl mercaptan was added.

The reactants of Examples 1-7 are summarized in Table 1.

                                      TABLE 1    __________________________________________________________________________                                   Chain    Initial                        Transfer    Feed, g      Initiator         Agent, g    (% of total            Pressure                 g          Second Feed                                   (% VAc + AA)    VAc + AA)            psi  APS (% of  g      n-dodecyl    Ex.      VAc   Ethylene                 VAc + AA)                       NaHCO.sub.3                            VAc AA mercaptan    __________________________________________________________________________    1 75 (5.5)            1000 40 (2.9)                       17   1225.2                                57 9.5 (0.6)    1A      2400 (5.1)            900  700 (1.5)                       466  43,100                                1825                                   302 (0.6)    1B      3850 (8.4)            900  500 (1.1)                       320  39,900                                1820                                   290 (0.6)    2 75 (5.5)            1000 40 (2.9)                       17   1225.2                                57 3.8 (0.3)    3 75 (4.5)            700  40 (2.9)                       17   1525.1                                57 3.8 (0.2)    4 75 (4.2)            550  40 (2.4)                       17   1650.0                                57 3.8 (Q.2)    5 75 (4.8)            1000 40 (2.2)                       17   1503.4                                0  3.8 (0.2)    6 75 (5.5)            1050 24.5 (1.8)                       13.6 1225.2                                57 0    7 75 (5.5)            1150 12.6 (0.9)                       7.0  1225.2                                57 0    __________________________________________________________________________     VAc: vinyl acetate     APS: ammonium persulfate     NaHCO.sub.3 : sodium bicarbonate     AA: acrylic acid

Comparative Examples 8-12 COMMERCIAL VAE ADHESIVES

For comparison purposes, known vinyl acetate-ethylene adhesives weregiven same tests as samples from Examples 1-7. Airflex® 400, Airflex®401, Airflex® Airflex® 465, and Airflex® 7200 emulsion copolymers wereused in each of the comparative examples. Airflex 400 (A-400) is a VAEcopolymer latex having a Tg of 0° C., Airflex 401 (A-401) is a VAE latexhaving a Tg of -15.0° C., Airflex 426 (A-426) is a carboxylfunctionalized VAE latex having a Tg of 0° C., Airflex 465 (A465) is ahigh solids VAE latex having a Tg of -5° C., and Airflex 7200 Dev(A-7200) is a high solids VAE latex having a Tg of 0° C. All areavailable from Air Products and Chemicals, Inc.

The emulsion polymers of Examples 8-12 were prepared using typicalemulsion polymerization techniques in which the monomers are added inbatch to the reactor at the beginning of the process, the level ofinitiator is 0.01 to 1%, preferably 0.01 to 0.5%, based on the weight ofvinyl acetate, and a chain transfer agent is not used.

The physical properties of the latexes of Examples 1 through 12 areshown in Table 2.

                                      TABLE 2    __________________________________________________________________________           Approx      Tg % THF                               % Non-                                    Viscosity*    Ex.      Polymer           Wt %               Mn  Mw  °C.                          Insolubles                               Volatiles                                    cps    __________________________________________________________________________    1 VA/E/AA           62/35/3               10,100                   82,000                       -25                          40   58.7 3370    1A      VA/E/AA           65/32/3               10,000                   113,000                       -18          2960    1B      VA/E/AA           67/30/3               4,950                   145,000                       -15          2580    2 VA/E/AA           62/35/3               9,100                   89,000                       -24                          34   57.4 3010    3 VA/E/AA           70/27/3               9,700                   86,000                       -14                          40   59.8 8790    4 VA/E/AA           76/21/3               9,900                   113,000                       -2 37   56.1 4500    5 VA/E 68/32               9,300                   87,000                       -22                          48   56.7 260    6 VA/E/AA           65/32/3     -21          874    7 VA/E/AA          -20          372    8 A-400           ˜80/20               60,000                   300,000                       0            1800-2700      VA/E    9 A-401           ˜70/30               40,000                   200,000                       -15          1300-2200      VA/E    10      A-426    20,000                   250,000                       0            1000-1800      VA/E/AA    11      A-465           ˜78/22                       -5           800-1300      VA/E    12      A-7200           ˜80/20               20,000                   600,000                       0            1500-3000      VA/E    __________________________________________________________________________     VA/E/AA: Vinyl Acetate/Ethylene/Acrylic Acid     *Brookfield RVF viscometer, #4 spindle, 20 rpm, 25° C.

Peel strength was measured for Examples 1-12 and tensile dynamicmechanical properties were measured for Examples 1A, 1B, and 4-12.

Dynamic mechanical testing of the polymer samples was accomplished usingthe following procedure. ASTM-D-4065-94 and ASTM-D-5026-94 were used asguidelines for this procedure. Each polymer emulsion was cast as a filmand allowed to dry a minimum of several days at ambient conditions. Thedry film thickness was typically in the range of 1 to 1.5 mm. Thespecimens used for testing were die cut from the film and were about 6.3mm wide and 30 mm long. The specimens were tested on a Rheometrics SolidAnalyzer (RSA II), from Rheometric Scientific, Inc., to obtain thetensile dynamic mechanical properties. Data were obtained every 6° C.over a -100° to 200° C. range using a fiber/film fixture and adeformation frequency of 6.28 rad/sec. To help ensure linearviscoelastic conditions, the applied strains were typically 0.05% in theglassy region and up to 1% in the rubbery region. A soak time of oneminute was used at each temperature to ensure isothermal conditions. Foreach temperature, the RSA II calculated the tensile storage modulus(E'), tensile loss modulus (E"), and tangent delta (tan d) based on thewidth, thickness and length of the sample.

The following method was used to determine peel strength of theadhesives. In this standard test, cloth instead of paper is laminated toPET and MPET because paper can break down or tear before the peelstrength of the adhesive is reached:

Cotton poplin cloth (mercerized, style 407) and the polymeric substratewere conditioned in a controlled environment room (23±1.0° C. and50±2.0% relative humidity), at least 24 hours prior to use. An 8-inch by8-inch square of polymeric substrate was then cut into 1-inch strips inthe machine direction, using a precision die cutter. A 6-inch wideswatch of cotton cloth was then cut from the roll in a cross-machinedirection. One strip of the control substrate was coated in thecontrolled-environment room. Two polymeric strips were placed, test sideup, on a piece of paper on a hard, smooth surface. The paper and thepolymeric strip were secured at the top with a 3-inch binding clip and awire-wound applicator was centered at the top of the strip. Using atongue blade, a small amount of adhesive was applied to the polymericstrip just below a No. 40 wire-wound applicator. The applicator wasdrawn down without using pressure, the cotton cloth was immediatelyplaced on top of the coated strip, and a 7-lb roller was passed over itonce. The paper was removed and the cloth cut in half, withapproximately equal distance between the two polymeric strips and atleast 1/2-inch of cloth on each side of the strips. The lamination wasplaced in the controlled-environment room for at least 16 hours prior totesting. To test, the lamination was placed in an Instron Tester withthe cloth strip in the top jaw and the polymeric strip in the bottomjaw. The Instron was set on a 10-lb. scale, with the crosshead speed at2 inches per minute and the chart speed at 1 inch per minute. The stripswere T-peeled at a 180° angle and the reading in pounds/linear inch(pli) recorded. The average peel strength was calculated by computing anaverage reading pounds/linear inch for all of the strips tested.

The following procedure was used to determine creep resistance of theadhesives by subjecting cloth laminations to an elevated temperatureunder a static load:

Masking tape was used to secure the top and bottom of the 9-inch side of11 inch by 9-inch section of cotton poplin to a hard, flat, smoothsurface. A No. 10 wire-wound applicator was centered at the top of theshorter side of cloth and a line of emulsion was applied within 1 inchof the cloth's side edges, just below the wire-wound applicator. Theapplicator was drawn down and the timer started. After 60 seconds, a No.40 wire-wound applicator was placed at the top, emulsion was applied,and the applicator drawn down. The tape was removed from the bottomedge. After 120 seconds, the laminated cloth was folded in half,aligning the top and bottom edges. One pass was made with a 7-lb rollerfrom top to bottom on one half and then the other. The lamination wasallowed to air dry overnight. After drying, the laminations were cutinto six 1-inch-wide strips parallel to the short side and 4 stripsselected from each lamination. The laminated edges were pulledapproximately 1 inch apart and the intact bond line was marked. The freeedge of the strip was folded inward and clamped with a 1-inch bindingclip. The strips were suspended from the ceiling of an oven set at 170°F. and a 500 g weight was placed on the free clip. At the start of thetest, the oven was checked frequently for rapid separation of thelamination. The strips were removed from the oven before the weighttouched the oven floor and the time of removal was recorded. When thestrips were removed from the oven, the intact bond line was marked andthe remainder of the lamination separated. The distance between thefirst and the second bond lines was measured. The creep resistance wascalculated by dividing the distance of the bond lines (in millimeters)by the time (in minutes) for each sample in the oven and averaging theresults of the strips tested.

Data on tensile storage modulus, peel strength, and creep resistance arepresented in Table 3.

                                      TABLE 3    __________________________________________________________________________                    Tensile                    Storage    Polymer         Modulus*    Approx.         (6.28 rad/sec)                                Peel                                    Peel    Wt %    Tg Viscosity                    dynes/cm.sup.2 × 10.sup.6                                (PET)                                    (MPET)                                        Creep    Ex      VA/E/AA            °C.               cps  23° C.                        45° C                            70° C                                pli pli (mm/min)    __________________________________________________________________________    1 VA/E/AA            -24               3010 --  --  --  1.22                                    1.16                                        31.9      62/3503    1A      VA/E/AA            -18               2960 5.5 0.59                            0.12                                1.42                                    1.04                                        46.8      65/32/3    1B      VA/E/AA            -15               2580 9.1 1.7 0.29                                1.44                                    0.90                                        30.0      67/30/3    2 VA/E/AA            -25               3370 --  --  --  1.10                                    1.14                                        28.6      62/35/3    3 VA/E/AA            -14               8790 --  --  --  1.30                                    1.15                                        20.1      60/27/3    4 VA/E/AA            -2 4500 6.6 0.68                            0.1 1.29                                    0.97                                        12.1      76/21/3    5 VA/E  -22               260  3.3 0.4 0.079                                1.01                                    0.87                                        --      68/32    6 VA/E/AA            -21               874  6.2 1.6 0.47                                0.86                                    0.71                                        19.1      65/32/3    7 VA/E/AA            -20               372  7.7 2.0 0.60                                0.59                                    0.51                                        11.7      65/32    8 A-400 0  1800-                    510 130 19  0.13                                    0.66                                        0.06      VA/E 80/20               2700    9 A-401 -15               1300-                    440 120 19  0.41                                    0.62                                        0.11      VA/E 70/30               2200    10      A-426 0  1000-                    31  14  6   0.29                                    0.66                                        0.07      VA/E/AA  1800      80/20/    11      A-465 -5 800-1300                    32  13  4.8 0.43                                    0.48                                        --      VA/E 78/22    12      A-7200            0  1500-                    27  11  4.6 0.31                                    0.54                                        0.10      VA/E     3000      ˜80/20    __________________________________________________________________________     *Cast film of emulsion polymer.     VA/E  Vinyl acetateethylene polymer emulsion.     VA/E/AA  Vinyl acetateethylene-acrylic acid polymer emulsion.

A graphical representation of the tensile storage modulus, as measuredfor Examples 1A and 1B and 4-12, is shown in FIG. 1. FIG. 2 presents amagnification of the area representing the tensile storage modulus ofthe polymers of this invention. The solid lines encompass the possibletensile storage modulus, at a frequency of 6.28 rad/sec, and the brokenlines encompass the preferred tensile storage modulus, at temperaturesbetween 23° C. and 70° C., of a cast film of an aqueous VAE emulsionpolymer of this invention.

In order to meet requirements of adhesion to difficult to adheresurfaces, an adhesive with a peel strength of at least about 0.5 pli isacceptable and peel of at least about 0.9 pli is preferred. The data inTable 3 show that, when applied to an untreated hard to adhere surface,such as PET or MPET, VAE emulsion polymer adhesives of Examples 1-6 hadsignificantly improved peel strength compared to known commercial VAEemulsion polymer adhesives (Comparative Examples 8-12.) The adhesives ofExamples 1-7 had a peel strength of at least about 0.5 pli on PET andMPET, the adhesives of Examples 1-6 had a peel strength of at leastabout 0.9 on PET, and the adhesives of Examples 1-5 had a peel strengthof at least 0.9 on PET and MPET. In contrast, the commercial VAEadhesives of Comparative Examples 8-12 had a peel strength that did notexceed 0.43 on PET and did not exceed 0.66 on MPET. The tensile storagemodulus of Examples 1-7 were at least about 1 order of magnitude lessthan the tensile storage modulus of the known VAE adhesives ofComparative Examples 8-12.

These data show that production of aqueous emulsion VAE polymers withsimilar ratios of vinyl acetate to ethylene, similar glass transitiontemperatures, and similar viscosities, leads to entirely differentpolymers as shown by the tensile storage modulus, peel strength, andcreep resistance. Without intending to be bound by theory, it isbelieved that the differences in methods of preparation of the VAEadhesives of Examples 1-7, compared to the commercial VAE adhesives ofComparative Examples 8-12, result in different aqueous VAE emulsionpolymers having tensile storage moduli that are considerably lower thanthe VAE adhesives of Comparative Examples 8-12. The preparative methoddescribed in this application leads to a VAE emulsion polymer withconsiderably improved peel strength compared to known VAE emulsionpolymers. The distinctive features of the preparative method for aqueousVAE emulsion polymers of this invention are: addition of no more thanabout 15% vinyl acetate at the beginning of the polymerization and delayaddition of the remainder of the vinyl acetate during polymerization,use of at least 0.5 wt % initiator, based on total monomers, andaddition of a chain transfer agent. In contrast, typical VAE emulsionpolymerization methods add all monomers, in batch, at the beginning ofthe polymerization, preferably use no more than about 0.5 wt %initiator, and do not add chain transfer agent.

The VAE adhesives of Examples 1-4 contained about 3% acrylic acid andshowed the most significant improvement in peel strength (at least 0.9pli on PET and MPET), when compared to the commercial VAE adhesives ofComparative Examples 8-12 (no more than 0.4 pli on PET and no more than0.66 pli on MPET); especially compared to the adhesive of ComparativeExample 10 (0.29 pli on PET and 0.66 pli on MPET) in which the polymercontains carboxyl functionality in the form of acrylic acid. Inaddition, Examples 1A, 1B and 4 showed significant differences intensile storage modulus (5.5 to 9.1×10⁶ dynes/cm² at 23° C. and 0.1 to0.29×10⁶ dynes/cm² at 70° C.), compared to Comparative Examples 8-12 (27to 510×10⁶ dynes/cm² at 23° C. and 4.6 to 19×10⁶ dynes/cm² at 70° C.).

A reduction of the chain transfer agent from 9.5 g to 3.8 g (Example 2compared to Example 1), and maintaining the level of initiator at 2.9 wt%, did not have a significant effect on peel strength. Peel remainedabout 1 pli on PET and MPET.

The tensile storage modulus of Example 1A and 1B (scale-ups ofExample 1) was similar to the tensile storage modulus of Example 4, eventhough 0.6 wt % of chain transfer agent was used in Examples 1A and 1B,compared to 0.2 wt % in Example 4. The initiator level was 2.4 wt % inExample 4, compared to 1.5 and 1.1 wt % in Examples 1A and 1B,respectively.

Examples 3 and 4 showed that reducing pressure to 700 psig (Example 3)or 550 psig (Example 4) and increasing the vinyl acetate concentrationin the second delay-feed, during emulsion polymerization, did notsignificantly affect peel strength (It remained above 1 pli on PET andMPET.) In addition, the tensile storage modulus of Example 4 was 6.6×10⁶dynes/cm² at 23° C., and 0.1×10⁶ dynes/cm² at 70° C.

The VAE polymer of Example 5 did not contain carboxyl functionality, butthe peel strength is maintained at about 1 pli for PET and 0.9 pli forMPET. Example 5 and Comparative Example 9 have similar Tg values;however, the tensile storage modulus at 23° C. and 70° C. is at leasttwo orders of magnitude lower for Example 5 compared to thecorresponding values for Comparative Example 9. In addition, the peelstrength of Example 5 is much higher than Comparative Example 9 (0.41pli on PET and 0.62 pli on MPET).

The adhesive of Comparative Example 10 contains carboxyl functionality;however, the peel is significantly lower (0.29 on PET and 0.66 on MPET)than the peel shown with Examples 1-4 (at least 0.9 pli on PET and MPET)which contain 3% acrylic acid. The peel strength of Comparative Example10 is similar to other commercial VAE adhesives that contain no carboxylgroups (comparative Examples 8-9 and 11-12.) In addition, the tensilestorage modulus at 23° C. and 70° C. of Comparative Example 10 is 31×10⁶dynes/cm² and 6×10⁶ dynes/cm², respectively, compared to thecorresponding tensile storage moduli of Example 4 (6.6×10⁶ and 0.1×10⁶,respectively). The tensile storage modulus is considerably different,even though glass transition temperatures are similar. The Tg forComparative Example 10 is 0 and the Tg for Example 4 is -5.

Example 6 showed the effects of reducing the amount of initiator to 1.8%of the total monomers, instead of 2.2%, or more, in Examples 1-5, andnot adding a chain transfer agent. The peel strength of the adhesive ofExample 6 (0.86 pli on PET and 0.71 pli on MPET) was less than the peelstrength of Examples 1-5, but was better than the peel strength of theadhesives of Comparative Examples 8-12 (0.13 to 0.43 on PET and 0.48 to0.66 on MPET). The tensile storage modulus for Example 6 was 6.2×10⁶dynes/cm² at 23° C., and 0.47 dynes/cm² at 70° C., compared to 27 to510×10⁶ dynes/cm² at 23° C., and 4.6 to 19 dynes/cm² at 70° C. forComparative Examples 8-12.

Example 7 showed the effects of reducing the amount of initiator to 0.9%of the total monomers and not adding chain transfer agent. The peelstrength (0.59 on PET and 0.51 on MPET) was not as good as in Examples1-6; however, it was within the acceptable range for difficult to bondsurfaces and the tensile storage modulus was 7.7×10⁶ dynes/cm² at 23° C.and 0.60×10⁶ dynes/cm² at 70° C.

The data of Examples 6 and 7 showed that, in the absence of a chaintransfer agent, reducing the initiator below about 2 wt % affects thepeel strength, especially when the initiator was reduced below about 1wt %.

These data indicate that Tg, viscosity, or the presence of carboxylgroups, in aqueous emulsion polymers, are not, by themselves, accuratepredictors of adhesive properties, even though they are known to beimportant properties in evaluating adhesives for difficult to bondsurfaces. However, tensile storage modulus provided an excellentpredictor of adhesive properties, such as peel strength.

The VAE emulsion polymers of this invention exhibit unexpected excellentpeel strength compared to known VAE emulsion polymers. This propertymakes them particularly useful in specific adhesive applications, suchas, cartons, flexible food packaging, film laminating, carton formingand sealing, plastic bottle labeling, carton windows, and collating.

We claim:
 1. An aqueous emulsion polymer adhesive comprising 55-80 wt %vinyl acetate, 15-45 wt % ethylene, and 0-30 wt % of one or moreadditional ethylenically unsaturated copolymerizable monomer, based onthe total weight of monomers, wherein a cast film of the adhesive has atensile storage modulus, at a test frequency of 6.28 rad/sec, within anarea encompassed by data points: 1×10⁵ and 2×10⁷ dynes/cm² at 23° C.,and 1×10³ and 2×10⁶ dynes/cm² at 70° C.
 2. The adhesive of claim 1comprising 60-75 wt % vinyl acetate, 20-40 wt % ethylene, and 1-5 wt %of one or more additional ethylenically unsaturated copolymerizablemonomer and having a tensile storage modulus within an area encompassedby data points: 1×10⁶ and 1.5×10⁷ dynes/cm² at 23° C., and 3×10⁴ and9×10⁵ dynes/cm² at 70° C.
 3. The adhesive of claim 2, wherein the one ormore additional ethylenically unsaturated copolymerizable monomer is acarboxyl-containing compound.
 4. The adhesive of claim 3 wherein theadhesive is prepared by aqueous emulsion polymerization in the presenceof a stabilizing system comprising 0.5-5 wt % polyvinyl alcohol and 1-4wt % surfactant, based on vinyl acetate monomer.
 5. The adhesive ofclaim 4 wherein the polyvinyl alcohol has a degree of polymerizationranging 50 to 3000 and is 75 to 99+ mole % hydrolyzed and the surfactantis a polyalkoxylated compound.
 6. The adhesive of claim 5 wherein thepolyvinyl alcohol has a degree of polymerization ranging from 100 to1500 and is 85-90 mole % hydrolyzed and the polyalkoxylated compound isan alkylphenoxy-poly(ethyleneoxy)ethanol having alkyl groups containingfrom about 7-18 carbon atoms, and having from about 4 to 100 ethyleneoxyunits.
 7. The adhesive of claim 6 wherein the polyvinyl alcohol has adegree of polymerization ranging from 200 to 1000 and is 87-89 mole %hydrolyzed and the polyalkoxylated compound is nonylphenolethoxylate. 8.The adhesive of claim 7 wherein a chain transfer agent is added duringaqueous emulsion polymerization.
 9. The adhesive of claim 8 wherein thechain transfer agent is added at a level of 0.02 to 2 wt %, based on thetotal weight of monomers.
 10. An aqueous emulsion polymer adhesiveconsisting essentially of 55-80 wt % vinyl acetate, 15-45 wt % ethylene,and 0-30 wt % of one or more additional ethylenically unsaturatedcopolymerizable monomer, based on the total weight of monomers, whereina cast film of the adhesive has a tensile storage modulus, at a testfrequency of 6.28 rad/sec, within an area encompassed by data points:1×10⁵ and 2×10⁷ dynes/cm² at 23° C., and 1×10³ and 2×10⁶ dynes/cm² at70° C.
 11. The adhesive of claim 10 consisting essentially of 60-75 wt %vinyl acetate, 20-40 wt % ethylene, and 1-5 wt % one or more additionalethylenically unsaturated copolymerizable monomer and having a tensilestorage modulus within an area encompassed by data points: 1×10⁶ and1.5×10⁷ dynes/cm² at 23° C., and 3×10⁴ and 9×10⁵ dynes/cm² at 70° C. 12.The adhesive of claim 11, wherein the one or more additionalethylenically unsaturated copolymerizable monomer is acarboxyl-containing compound.
 13. The adhesive of claim 12 wherein theadhesive is prepared by aqueous emulsion polymerization in the presenceof a stabilizing system comprising 0.5-5 wt % polyvinyl alcohol and 1-4wt % surfactant.
 14. The adhesive of claim 13 wherein the polyvinylalcohol has a degree of polymerization ranging 50 to 3000 and is 75 to99+ mole % hydrolyzed and the surfactant is a polyalkoxylated compound.15. The adhesive of claim 14 wherein the polyvinyl alcohol has a degreeof polymerization ranging from 100 to 1500 and is 85-90 mole %hydrolyzed and the polyalkoxylated compound is analkylphenoxy-poly(ethyleneoxy)ethanol having alkyl groups containingfrom about 7-18 carbon atoms, and having from about 4 to 100 ethyleneoxyunits.
 16. The adhesive of claim 15 wherein the polyvinyl alcohol has adegree of polymerization ranging from 200 to 1000 and is 87-89 mole %hydrolyzed and the polyalkoxylated compound is nonylphenolethoxylate.17. The adhesive of claim 16 wherein a chain transfer agent is addedduring aqueous emulsion polymerization.
 18. The adhesive of claim 17wherein the chain transfer agent is added at a level of 0.02 to 2 wt %,based on the total weight of monomers.